1. Field of the Invention
The present invention relates generally to a method of and apparatus for reading out radiation image data, and more particularly to a method of and apparatus for reading out the radiation image data stored on a stimulable phosphor sheet by use of a line sensor formed of one or more CCD sensors.
2. Description of the Related Art
There are in wide use today technologies for obtaining an image signal of a radiation image by use of a stimulable phosphor sheet. Such stimulable phosphor sheets have a layer containing stimulable phosphor, which are supported on a sheet-type substrate, that cumulatively store a portion of the radiation passing through the body of a subject, such as the body of a human being, and emit a stimulated emission corresponding to the radiation energy stored thereon upon irradiation thereof by a stimulating light such as visible light or a laser beam. In such technologies, radiation image energy that has been once cumulatively stored on a stimulable phosphor sheet is caused to be emitted as a stimulated emission by scanning each pixel of the stimulable phosphor sheet in order with a deflected laser beam or other stimulating light; the stimulable emission is then photoelectrically read out in order by a photoelectrical readout means and an image signal is obtained. On the other hand, after the radiation image signal has been readout, the stimulable phosphor sheet is irradiated with an erasing-light that causes the radiation energy remaining thereon to be dissipated, and the sheet can be reused for the same operation of storing and reading out a radiation image (refer to Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 55(1980)-116340, 56(1981)-104645, and etc.).
In addition, a system has been proposed in which, in order to raise the efficiency of quantum detection upon formation of the radiation image, that is, the absorption rate of the radiation energy, the efficiency of the output of the stimulated emission, and the efficiency of detecting the stimulated emission, the radiation absorbing function and the energy accumulating function occurring in a conventional stimulable phosphor are separated, and a phosphor having superior radiation absorption properties and a phosphor showing superior response to stimulable emission are each used separately for absorbing radiation and accumulating radiation image data, respectively: using the phosphor superior in absorbing radiation (radiation absorption use phosphor), the radiation is absorbed and light in the visible to UV spectra is emitted; this emitted light is absorbed and the energy therefrom accumulated using the phosphor showing superior response to stimulated emission (dedicated accumulation-use phosphor); this accumulated energy is stimulated by the irradiation thereof with a stimulating light in the visible to infrared range, causing a stimulated emission to be emitted; and the light of the stimulated emission is photoelectrically readout sequentially by a photoelectrical readout means and a image signal is obtained (refer to Japanese Patent Application No. 11(1999)-372978).
The image signal obtained by these systems is subjected to gradation processing, frequency processing, and etc. image processing applied in diagnostic reading, and after being subjected to such processing, the image signal is recorded on film as a diagnostic image (final image), or displayed on a high-resolution CRT display for diagnostic use by a doctor, etc. On the other hand, if the stimulable phosphor sheet is irradiated by an erasing-light, the energy remaining on the stimulable phosphor sheet after the radiation image data once stored thereon has been readout is erased, and it becomes possible to again cumulatively record radiation image data on the sheet, and it possible to repeatedly use the sheet for such operations.
Here, according to the radiation image data readout apparatus occurring in the radiation image recording and reproducing systems described above, a configuration has been proposed with a view to shortening the readout time for the stimulated emission, reducing the cast and the size of the apparatus: a line-light source is used as the stimulating-light source for irradiating the stimulable phosphor sheet with a line-shaped stimulating-light; a line sensor formed of a plurality of photoelectric converting elements arranged along the lengthwise direction (hereinafter referred to as the main-scanning direction) of the line-shaped portion of the stimulable phosphor sheet irradiated by the stimulating-light emitted from the line-light source; and a scanning apparatus moving in a direction substantially perpendicular to the lengthwise direction of aforementioned line-shaped portion (hereinafter referred to as the sub-scanning direction), from one side to another of the sheet relative to the line-light source and the line sensor (refer to Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, 1(1989)-101540, etc.).
However, because of remarkable performance advances such as increased pixel density, improved sensitivity, noise reduction, reduction in image size, and etc. attainable due to innovations in micro-processing used in the manufacture of semiconductors in recent years, CCD sensors are frequently used as the photoelectric converting elements that form the line sensor described above.
However, there is a limit inherent in a line sensor formed of one or more CCD sensors regarding the quantity of charge that can be accumulated before the charge saturation quantity is reached, and because each CCD element of each CCD sensor can not detect quantities of light above a fixed quantity, for portions of a stimulable phosphor sheet where a large amount of radiation data (radiation energy) have been accumulated (high-radiation zones), a precise measurement thereof cannot be obtained, and a problem arises in that the range with respect to the quantity of light possible of being read out (dynamic range) when a series of CCD sensors are used as a line sensor is said to be narrow.
Therefore, as described in PCT International Publication No. WO99/28765, a method for expanding the dynamic range of the line sensor has been proposed, wherein, readout of a quantity of light below the saturated charge quantity is performed a plurality of times, and by adding together the data obtained by this plurality of readout operations, each CCD element does not become saturated.
However, in the method proposed in PCT International Publication No. WO99/28765, because the actual quantity of radiation energy stored on each portion of a stimulable phosphor sheet is not grasped and readout of low-quantity of light is simply performed repeatedly for high-radiation zones, each CCD element may become substantially saturated, and there is a possibility that it will not be possible to read out accurate data. Further, because the noise occurring when readout is performed n times is n times as large as the noise occurring when readout is performed only once, for portions of a stimulable phosphor sheet which have accumulated only a small quantity of radiation energy (low-radiation zones), the higher the number of times readout is performed, the lower the S/N ratio becomes, and a problem arises in that the quality of the image is thereby deteriorated.
The present invention has been developed in consideration of the circumstances described above, and it is a primary object of the present invention to provide a method and apparatus for always performing read out of radiation image data at a detected light quantity range below the charge saturation quantity of the each CCD element of CCD sensors forming a line sensor, which controls the increase of noise due to performing readout a plurality of times and expands the dynamic range of the line sensor.
The radiation image data readout method according to the present invention comprises: irradiating with a stimulating-light a portion of the face of a stimulable phosphor sheet on which radiation image data has been stored; receiving and photoelectrically converting, by use of a line sensor formed of one or more CCD sensors each divided into a plurality of pixel zones extending along the lengthwise direction of the irradiated portion of the stimulable phosphor sheet, the stimulated emission emitted from the portion of the stimulable phosphor sheet that has been irradiated in a line-shape by the stimulating-light, or the portion of the back side of said stimulable phosphor sheet corresponding to the portion that has been irradiated by the stimulating-light; moving the line-light source emitting the stimulating-light and the line sensor relatively, in a direction different from that of aforementioned lengthwise direction, from one end of the stimulable phosphor sheet to the other and sequentially reading out the output of the line sensor corresponding to each readout line according to aforementioned movement; and obtaining the output of each of the pixel zones at each position of the readout lines, wherein when readout is performed, the radiation energy stored on the readout line of each position is estimated, and based on said estimated quantity of radiation energy, the readout settings, which are formed of the line sensor charge accumulation time and the binning conditions, are set, and the readout lined are readout according to said readout conditions.
A fluorescent lamp, a cold cathode fluorescent lamp, an LED array, and etc. can be adopted as the line-light source. Also, the line-light source is not limited to being alight source element in the form of a line, such as aforementioned fluorescent lamp, etc., but can be a light source that emits the stimulating-light in a line form, including a broad are a laser beam, etc. The line-light emitted from the line-light source can be emitted in a continuous manner or can be a pulse light, which is repeatedly emitted and stopped, however, from the standpoint of reducing the noise level, it is desirable that a high-output pulse light is used.
In addition, the expression xe2x80x9cmoving the line-light source emitting the stimulating-light and the line sensor relatively, in a direction different from that of aforementioned lengthwise direction, from one end of the stimulable phosphor sheet to the otherxe2x80x9d refers to the direction substantially perpendicular to the lengthwise direction of the line-light source and the line sensor, that is to say, it is desirable that it be the direction of the short axis, however, it is not limited to being this direction; for example, the line-light source and the line sensor can be moved in a direction displaced from the direction perpendicular to the lengthwise direction of thereof in a diagonal direction, within the range in which the stimulated light can be irradiated over substantially the entire surface of the stimulable phosphor sheet in a zigzag motion etc.
Note that the line-light source and the line-light sensor can be provided on the same side of the sheet or on mutually opposite sides of the sheet. However, when a configuration in which they are each installed separately is adopted, the substrate of the sheet must be transparent to the stimulable emissions so that the stimulable emission can pass through the side opposite the side that from which the stimulating-light enters.
The expression xe2x80x9cone or more CCD sensorsxe2x80x9d constituting a line sensor refers to one or more solid-state detecting elements each formed of a plurality of CCD elements each covering a single pixel.
The line sensor according to the present invention is formed of one or more CCD sensors, i.e., of a CCD sensor or a plurality of CCD sensors arranged in a line. Note that for cases in which the line sensor is formed of a plurality of CCD sensors arranged in a line, the CCD sensors can be arranged in a single line along the lengthwise direction of aforementioned lengthwise direction, or can be arranged in each of the lengthwise direction and the direction perpendicular thereto in order to improve the focusing efficiency for the stimulated emission emitted from the stimulable phosphor sheet.
Of course, the stimulable phosphor sheet can be a sheet formed using normal stimulable phosphor that combines the properties of both stimulable phosphor that absorbs radiation and stimulable phosphor that accumulates radiation energy, that is, that stores radiation image data. However, because of the following reason, it is preferable that the stimulable phosphor sheet contains only dedicated accumulation-use stimulable phosphor. That is, according to a system proposed in Japanese Patent Application No. 11(1999) -372978, the radiation absorbing function and the energy accumulating function occurring in a conventional stimulable phosphor sheet are separated, and a phosphor having superior radiation absorption properties and a phosphor showing superior response to stimulable emission are each used separately for absorbing radiation and accumulating radiation image data, respectively: using the phosphor superior in absorbing radiation (radiation absorption-use phosphor), the radiation is absorbed and light in the visible to UV spectra is emitted; this emitted light is absorbed and the energy therefrom accumulated using the phosphor showing superior response to stimulated emission (dedicated accumulation-use phosphor); this accumulated energy is stimulated by the irradiation thereof with a stimulating light in the visible to infrared range, causing a stimulated emission to be emitted; and the stimulated emission is photoelectrically readout sequentially by a photoelectrical readout means and a image signal is obtained, wherein the efficiency of quantum detection upon formation of the radiation image, that is, the absorption rate of the radiation energy, the efficiency of the output of the stimulated emission, and the efficiency of detecting the stimulated emission can be raised overall.
Here, the dedicated accumulation-use phosphor absorbs the light in the UV to visible spectra emitted from the absorption-use phosphor, and accumulates the energy thereof as image data. However, because said emitted light in the UV to visible spectra has been emitted from the absorption-use phosphor upon the absorption of radiation thereby, the image data accumulated on the stimulable phosphor sheet is also to be referred to as radiation image data.
In addition, xe2x80x9creadout linexe2x80x9d refers to the line-shaped portion of the stimulable phosphor sheet that has been irradiated by the stimulating-light emitted in a line-shape and which is to be read out by the line sensor. According to the radiation image data readout method of the present invention, each time a readout line is to be read out, the quantity of radiation energy accumulated on said readout line is estimated, and based on said estimated quantity of radiation energy, the readout settings, which are formed of the charge accumulation time of the line sensor and the binning conditions, are set and readout of the readout line is performed according to said readout conditions; however, regarding the method of estimating the quantity of radiation energy accumulated on the readout line, more specifically, the first or second method described below can be used.
1. The quantity of radiation energy accumulated on the readout line is estimated based on the data obtained by reading out the preceding readout line.
Here, xe2x80x9cthe preceding readout linexe2x80x9d refers to the readout line directly preceding the readout line that is to be read out. According to this method, by making use of the fact that the quantity of radiation energy accumulated on each of two adjacent readout lines is close, because the data of the previous line forming a portion of the radiation image data stored on the stimulable phosphor sheet is used in estimating the quantity of radiation energy accumulated on the readout line that is to be read out, it is not necessary to obtain the data only for estimating the quantity of radiation energy accumulated on the readout line that is to be read out. Therefore, the time required for determining the readout settings can be shortened, and readout having a high possible efficiency can be provided.
2. The quantity of radiation energy accumulated on the readout line that is to be read out is estimated based on the data obtained by conducting a preliminary reading of said readout line.
Here, xe2x80x9cconducting a preliminary readingxe2x80x9d refers to conducting a readout process different from the actual read out (main read out), and comprises using a shortened readout time, stimulation by a stimulating-light of a relatively weak strength, or a shortened charge accumulation time to obtain the data for estimating the quantity of radiation energy accumulated on each readout line of the stimulable phosphor sheet. More specifically, after once conducting a preliminary reading over the entire stimulable phosphor sheet, the main read out can be performed. However, in order to improve the readout efficiency potential, it is preferable to perform the main read out of each readout line directly after conducting a preliminary reading of said each readout line. In this case, in order to improve the accuracy with which the quantity of radiation energy accumulated on the readout line is estimated, the preliminary reading and the main read out of each readout line can be performed for the readout line of exactly the same position (in this case, it is necessary to once stop the movement in the sub-scanning direction), because of the reason that the time required for conducting a preliminary reading is extremely short, if the movement in the sub-scanning direction is not stopped while the preliminary reading and the main readout are performed alternately, there is a slight difference in the position of the readout line of which the preliminary reading is performed and the readout line of which the main readout is performed. However, control of the scanning operation can be simplified and the readout efficiency is improved.
If this method is used, because the quantity of radiation energy accumulated on a readout line that is to be subjected to main read out (or a readout line that is slightly displaced from the readout line to be subjected to main read out) is estimated based on data obtained by conducting a preliminary reading of said readout line, the accuracy in estimating the quantity of accumulated radiation energy can be improved.
According to the present invention, the xe2x80x9creadout conditions for each readout linexe2x80x9d refer to the charge accumulation time of the line sensor and the binning conditions for the line sensor under which each of said readout lines is read out. According to the present invention, the charge accumulation time of the line sensor is determined, based on the quantity of radiation energy accumulated on a readout line that is to be readout, so that the quantity of light detected by each CCD element is smaller than the charge saturation quantity of the CCD element, even for the portion of the readout line on which the most radiation energy has been accumulated. In this way, because the quantity of light detected by each CCD element of the CCD sensors constituting the line sensor can be reliably made to be below the charge saturation quantity thereof, readout can be performed accurately, even for the high-radiation zones of the stimulable phosphor sheet.
In addition, for cases in which the line sensor is formed of a plurality of CCD sensors, if the charge accumulation time is determined so that the quantity of light detected at the readout position corresponding to each CCD element is below the charge saturation quantity of each CCD element, even for the portion of the readout line on which the most radiation energy has been accumulated, readout can be performed more efficiently. Thus, in this case, it is preferable that the charge accumulation time be set for each CCD sensor. Note that for cases in which the line sensor is formed of a plurality of CCD sensors arranged in a line in the direction along aforementioned lengthwise direction as well as in the direction perpendicular to said lengthwise direction, in order to provide for simplicity in control, it is preferable that a similar charge accumulation time be determined for the CCD sensors at the same position as that at which the stimulated emission is emitted.
Note that when determining the charge accumulation time, the quantity of detected light for each CCD element can be any quantity below the charge saturation quantity of the CCD element, however, because of the reason that the number of times readout is performed should be reduced to as few as possible in order to reduce noise, it is desirable that the charge accumulation time is determined so that readout be performed such that the quantity of detected light for each CCD element is close to the charge saturation quantity of the CCD element.
Further, xe2x80x9cbinningxe2x80x9d refers to a technique of not reading out the signal from each of a plurality of pixels zones of the CCD element one by one, but combining and reading out the signal from a plurality of pixel zones at one time. By performing binning, because the signals from a plurality of pixel zones are averaged and the data from said plurality of pixel zones formed based on said averaged signal, although a slight deterioration in resolution is incurred, because the number of times readout is performed can be reduced, the effectiveness in reducing noise and improving the readout speed is great, especially with respect to low-radiation zones.
The xe2x80x9cbinning conditionsxe2x80x9d, which constitute one of the readout conditions according to the present invention, refer to the number of pixel zones to be binned (the number of times of binning) for a readout line: according to the present invention, the line sensor binning conditions are determined so that, based on the estimated quantity of radiation energy accumulated on a readout line, the number of times binning is performed is smaller for the portions of the readout line on which more radiation energy has been accumulated, and the number of times binning is performed is larger for the portions of the readout line on which less radiation energy has been accumulated. Thus, after estimating the quantity of radiation energy accumulated on each position of the readout line, the noise is reduced and the readout speed improved with respect to the low-radiation zones, and the saturation of the accumulating circuit due to performing binning an excessive number of times can be prevented with respect to the high-radiation zones.
Still further, for cases in which the line sensor is formed of a plurality of CCD sensors, in order to more accurately perform readout, it is preferable that the binning conditions are determined for each CCD sensor at the same time that the charge accumulation time described above is determined.
The radiation image data readout apparatus according to the present invention comprises: a line-light source for irradiating with a stimulating-light emitted in a line-shape a portion of the surface of a stimulable phosphor sheet storing radiation image data; a line sensor formed of one or more CCD sensors for receiving and photoelectrically converting the stimulated emission emitted from the portion of the stimulable phosphor sheet that has been irradiated by the stimulating-light emitted in a line-shape or the portion of the rear face of the stimulable phosphor sheet corresponding to the portion of the stimulable phosphor sheet that has been irradiated by the stimulating-light emitted in a line-shape; a scanning means for moving the line-light source emitting the stimulating-light and the line sensor relatively, in a direction different from that of aforementioned lengthwise direction, from one end of the stimulable phosphor sheet to the other; and a readout means for sequentially reading out the output of the line sensor corresponding to each readout line according to aforementioned movement, and obtaining the output of each of the pixel zones at each position moved to; wherein the readout means further comprises a readout conditions controlling means for estimating the radiation energy stored on the readout line of each position moved to, and, based on said estimated quantity of radiation energy, setting the line sensor charge accumulation time and the binning conditions.
A stimulable phosphor sheet that is to be subjected to the radiation image readout method according to the present invention contains a stimulable phosphor capable of absorbing light in the wavelength range of UV to visible to accumulate the energy thereof, being stimulated by light in the wavelength range of visible to infrared, and emitting as stimulated emission the energy accumulated thereon.
The radiation image data readout apparatus according to the present invention further comprises a readout conditions controlling means for estimating the radiation energy stored on the readout line of each position moved to, and, based on said estimated quantity of radiation energy, setting the line sensor charge accumulation time and the binning conditions: more specifically, it is preferable that the readout conditions controlling means, in order to shorten the time required for determining the readout conditions, makes use of the fact that the quantity of radiation energy accumulated on each of two adjacent readout lines is close and estimates, based on the data read out from the preceding readout line, the quantity of radiation energy accumulated on the readout line that is to be read out; or, in order to improve the accuracy in estimating the quantity of radiation energy, conducts a preliminary reading of the readout line to be read out, and estimates the quantity of radiation accumulated thereon based on the data obtained by said preliminary reading.
Regarding the charge accumulation time, which is one of the readout conditions for each readout line, based on the estimated quantity of radiation energy, the readout conditions controlling means determines, for each readout line, the charge accumulation time of the line sensor so that the quantity of light detected for each CCD element is smaller than the charge saturation quantity of the CCD element, even for the portion of the readout line on which the most radiation energy has been accumulated. In this way, because it is possible for each CCD element of the CCD sensors constituting the line sensor to reliably detect a quantity of light in the range smaller than the charge saturation quantity, accurate read out can be performed even for the high-radiation zones of a stimulable phosphor sheet.
In addition, for cases in which the line sensor is formed of a plurality of CCD sensors, it is preferable that the readout conditions controlling means controls the readout conditions by separately setting the charge accumulation time for each CCD sensor of each readout line so that the quantity of light detected by each CCD element corresponding to a position on the readout line is smaller than the charge saturation quantity of the CCD element, even for the portion of the readout line on which the most radiation energy has been accumulated. In this way, readout can be performed still more efficiently. Note that for cases in which the line sensor is formed of a plurality of CCD sensors arranged in a line in the direction along aforementioned lengthwise direction as well as in the direction perpendicular to said lengthwise direction, in order to provide for simplicity in control, it is preferable that a similar charge accumulation time be determined for the CCD sensors at the same position as that at which the stimulated emission is emitted.
Note that when determining the charge accumulation time, the quantity of detected for each CCD element can be any quantity below the charge saturation quantity of the CCD element, however, in order to reduce the number of times readout is performed, it is desirable that the readout conditions controlling means determines the charge accumulation time so that readout is performed such that the quantity of detected light is close to the charge saturation quantity of the CCD element.
In addition, according to the present invention, after the readout conditions controlling means estimates the quantity of radiation energy accumulated on each position of the readout line that is to be read out, with respect to performing read out of the low-radiation portions of the readout line, the noise can be reduced and the readout speed improved, and with respect to the high-radiation portions of the readout line, in order to prevent the saturation of the binning accumulation circuit due to performing binning an excess number of times, it is preferable that the binning conditions of the line sensor are set, based on the estimated quantity of radiation energy accumulated on the readout line, so that the number of times binning is performed is made smaller for the portion of the readout line on which more radiation energy has been accumulated and the number of times binning processing is performed is made larger for the portion of the readout line on which less radiation energy has been accumulated.
Further, in the same way as occurs when the charge accumulation time is to be set as described above, for cases in which the line sensor is formed of a plurality of CCD sensors, in order to more accurately perform readout, it is preferable that the readout conditions controlling means determines the binning conditions separately for each CCD sensor.
According to the radiation image data readout method and apparatus therefor according to the present invention, when reading out each readout line, because the radiation energy accumulated on each said readout line is estimated, and based on said estimated quantity of radiation energy, the charge accumulation time is determined and the readout line is readout, readout can be performed so that each CCD element of the CCD sensors forming the line sensor always detect a quantity of light smaller than the charge saturation quantity of said CCD element. Further, when setting the charge accumulation time, because the quantity of radiation energy to be readout that has been accumulated on the readout line is estimated, it becomes possible to determine the charge accumulation time of each CCD sensor so that readout of the readout line is performed in a manner that the quantity of detected light for each CCD element is as close as possible to the charge saturation quantity of the CCD element, and the number of times readout is performed can be reduced.
In addition, according to the radiation image data readout method and apparatus therefor according to the present invention, when reading out each readout line, because the radiation energy accumulated on each said readout line is estimated, and based on said estimated quantity of radiation energy, the binning conditions of the line sensor are determined: for low-radiation zones of the readout line that is to be read out, the number of times binning is performed can be made large, and for high-radiation zones, the number of times binning is performed can be made small; whereby the noise can be reduced and the readout speed can be improved for readout of the low-radiation zones, and the saturation of the binning accumulation circuit due to performing binning an excessive number of times can be prevented for readout of the high-radiation zones.
Note that according to the radiation image data readout method and apparatus therefor according to the present invention, when estimating the radiation energy accumulated on the readout line that is to be readout, if said estimating is based on the data obtained by conducting a preliminary reading on the readout line preceding the readout line that is to be read out, the time required to determine the readout conditions can be shortened and the readout speed improved. Further, if a preliminary reading is conducted for the readout line that is to be readout and the quantity of radiation energy accumulated on said readout line is estimated based on the data obtained by said preliminary reading, the accuracy with which the quantity of radiation energy stored on said readout line is estimated can be improved.
Further, according to the radiation image data readout method and apparatus therefor according to the present invention, for cases in which the line sensor is formed of a plurality of CCD sensors, because it is possible to determine the charge accumulation time and the binning conditions separately for each CCD sensor, readout can be performed still more efficiently.
Still further, if the stimulable phosphor sheet that is to be the subject of the radiation image data readout method and apparatus therefor according to the present invention contains the dedicated accumulation-use stimulable phosphor described above, a higher level of image resolution can be provided.